Method and device for analysing blushing in dispersion films

ABSTRACT

A process for parallel and automated evaluation of the blushing of multiple samples of dispersion films, comprising: providing an illuminated array of spatially separated samples of dispersion films, imaging the array of samples by means of CCD camera at different times and digitizing the images, automatically determining multiple brightness for each sample at different times from the different digitized images for at least a portion of the samples, determining values of a parameter characteristic of the kinetics of the blushing from the different brightnesses, and comparing the values of the characteristic parameter for individual samples.

The invention relates to a process and to a device for evaluating theblushing of dispersion films.

Blushing is when initially clear dispersion films become cloudy. It maytake place immediately after production of the film or when the filmlater comes into contact with water. Blushing is caused by separationphenomena in the dispersion film or an ingress of water droplets, whichlead to light scattering and opalescence.

The degree of blushing of dispersion films is usually determined usingspectrocolorimeters or opacimeters as well as the naked eye.

Visual inspection is subjective and varies with the individual observer.Nor is the determination of the kinetics of blushing with high precisionpossible by visual inspection. Spectrolorimeters and opacimeters arerelatively complicated and expensive instruments. A high samplethroughput of several hundred or several thousand samples in a shorttime is not achievable by the methods mentioned.

U.S. Pat. No. 4,072,426 discloses a process and a device for determiningthe reflective characteristics of coated surfaces. A converging lens isused therein to create a beam of parallel light rays from the lightemitted by a light source and directing it onto the reflecting surfaceto be investigated. The light which is regularly reflected by thesurface is focused by means of a second converging lens onto a firstreceiver, for example a light-sensitive resistor. The majority of thescattered light is masked out by a circular diaphragm surrounding thereceiver and the light passing through the ring opening is focused bymeans of a concave mirror located behind the diaphragm onto a secondreceiver disposed on the reverse of the first receiver.

EP-A 1 030 173 discloses a process for automatic inspection of a movingsurface, for example a metal film. This involves illuminating the movingsurface with two or more pulsed light sources from different directions,each at different times, the pulse frequency being >1 kHz. Theilluminated surface region is imaged as lines by means of a line scancamera in sync with the light pulses. The process serves to recognizesurface anomalies.

It is an object of the invention to find a process for evaluatingblushing of dispersion films which is simple to implement andfacilitates a high sample throughput.

We have found that this object is achieved by a process for parallel andautomated evaluation of the blushing of n samples of dispersion films,which comprises

-   (a) providing an illuminated array of n spatially separated samples    of the dispersion films,-   (b) imaging the array of n samples by means of a CCD camera at m+1    different times t₀ to t_(m) and digitizing the images,-   (c) automatically determining a plurality of brightnesses B(k,    t_(i)) for each sample at different times t_(i) selected from t₀ to    t_(m) from the m+1 different digitized images for at least a portion    of the n samples, where B(k, t_(i)) is the brightness of the kth of    n samples at the respective time t_(i) and is a measure for the    blushing of this sample at this time, by averaging over the    individual brightnesses of the individual pixels assigned to this    sample,-   (d) determining values P(k) of a parameter which is characteristic    of the kinetics of the blushing from the different brightnesses B(k,    t_(i)) for each sample for at least a portion of the n samples,    where P(k) is the value of the characteristic parameter for the kth    of the n samples.

For the purposes of the present invention, dispersion films are quitegenerally coating films based on polymer dispersions.

The process according to the invention employs a customarily rectangulararray of n spatially separated samples of the dispersion films havingareas of customarily from 1 mm² to 1 000 cm², preferably from 1 to 100cm². A different geometry of the array, for example an approximatelycircular arrangement of the samples, is possible. The number n of thesamples may, depending on the dimensions of the samples, be very large,for example, up to 10 000. The number n is at least two, customarilyfrom 2 to 1 000 and preferably from 10 to 1 000. The samples arepreferably rectangular and applied in a regular arrangement to atransparent or black substrate. For example, they may be prepared on atransparent foil such as a PET foil. The samples may also be arranged ina transparent sample holder having rectangular holes. The backgroundbelow the transparent substrate is preferably black.

The array is preferably illuminated by a plurality of evenly distributedlight sources, for example by two light sources arranged on oppositesides or by four light sources arranged on all four sides of arectangular array. Even illumination is preferably achieved by using ahighly diffuse light source, for example a white light lamp having atungsten wire incandescent bulb.

Customarily, the process of blushing is initiated by wetting the sampleswith water. To this end, the samples are arranged in an appropriatecontainer and the container is flooded with water. Blushing by wettingwith aqueous solutions for certain substances or by liquids other thanwater may also be evaluated. Blushing of the samples under air may alsobe evaluated. Even the reverse process, ie the reduction of blushingafter removal of water or the liquid, may also be evaluated.

Blushing of the dispersion films causes an increase in scattering of theincident light by the samples, which increases the brightness at thesample location.

The array of the n samples is imaged by means of a CCD camera at m+1different times t₀ to t_(m) and the images are digitized. m may be aslarge as desired, and is customarily from 10 to 1 000. The timeresolution of the measurement, ie. the time interval between individualimages depends on the likely rate of blushing and may, for example, be asecond or even an hour. Digital imaging electronics, for example a framegrabber interface, are used to store the individual images in digitalformat in the memory of a control computer, for example a PC.

From the m+1 different digitized images, a plurality of brightnessesB(k, t_(i)) at different times t_(i) selected from t₀ to t_(m) areautomatically determined for each sample for at least a portion of the nsamples. Preference is given to determining brightness values for eachof the n samples at each of the m+1 times t₀ to t_(m). This is sensiblebut not strictly necessary. Digitization of, for example, 8 bit assignsa brightness from 0 to 255 to each pixel. The brightness of a particularsample is determined by averaging the brightnesses of the pixelsassigned to this sample. Since the number of pixels of every image ofthe CCD camera is very large, customarily from 500 000 to 1 000 000,there will be several hundred pixels for every individual sample in anumber n of samples of, for example, 1 000. The averaging over manypixels still allows the measuring precision, even with a high number nof samples, to be high, ie distinctly higher than, for example, 8 bit.

Preference is given to carrying out the averaging after every imagingstep and before the next imaging step, ie in real time. Thisconsiderably reduces the data quantity to be stored.

Preference is given to using the digitized images of the sample array todetermine the brightnesses for a sample by averaging the brightnesses ofpixels which have been assigned a certain predefined area of interestwholly within this sample. The size of the area of interest may, forexample, correspond to a half or only a tenth of the sample area and mayalso vary from sample to sample. The location and size of the area ofinterest may be individually chosen before each measurement by theoperator at the control computer for each sample or else be predefinedaccording to a fixed pattern for all samples, while the latter requiresa strictly regular arrangement of the samples in the array.

Preference is given to normalizing the brightnesses to a black surfacewhich defines the zero value of blushing and to a white surface whichdefines the maximum value of blushing. To this end, the sample array isprovided with a black and a white reference plate. Normalizationcompensates for variations in the illumination intensity during themeasurement. A large number of samples may be analyzed by distributing aplurality of black and white reference plates in the array andnormalizing each time using the nearest reference plates.

The resulting variations of brightness with time may be used todetermine values P(k) of parameters for individual samples whichcharacterize the kinetics of blushing. These values may be used tocompare individual samples. For example, the variations of brightnesswith time may be defined by simple exponential functions, which give arate constant for the kinetics of blushing as a characteristicparameter. Other sensible characteristic parameters include, forexample, the time to 50% or 90% of a certain maximum brightness, or elsethe brightness which is attained after a predefined time.

The process according to the invention is a particularly simple processfor determining the kinetics of blushing which facilitates a high samplethroughput and, even at a high sample number, enables a sufficientmeasuring precision. It is therefore particularly suitable for couplingto combinatorial material research processes which comprise an automaticformulation of dispersions and an automatic coating of substrates. Suchcombinatorial material research processes typically involve a highthroughput of samples to be characterized and therefore rely on aprocess for fast characterization of samples. Analysis of the largenumber of individual values obtained in this way for the characteristicparameter or parameters for the kinetics of blushing or the inverseprocess may reveal a systematic dependence on the composition of thedispersion formulations and the production conditions and showstructure-effect relationships which allow the dispersions to beoptimized with respect to their blushing behavior among otherproperties. For example, the type and the quantity ratio of the monomerscontained in the dispersion polymers, the polymer architecture, theconcentration of the different polymers and the auxiliaries in thedispersions and the conditions during the production of the coatings maybe systematically varied.

The present invention accordingly also provides a device for paralleland automated evaluation of the blushing of dispersion films, whichcomprises:

-   (i) an array of n spatially separated samples of dispersion films;-   (ii) one or more diffuse light sources for even illumination of the    sample array;-   (iii) a CCD camera disposed above the sample array for digital    reproduction of the sample array;-   (iv) digital imaging electronics;-   (v) a control computer.

The invention is illustrated with reference to the drawings.

FIG. 1 shows a schematic of a suitable measuring arrangement in sideview.

FIG. 2 shows a schematic of the same measuring arrangement in plan view.

FIG. 3 shows examples of measured variations of brightness with time.

Samples of dispersion films 6 and 7 and reference plates 5 and 8 havingareas of from 1 mm² to 1 000 cm² are arranged in an array and placedunder a CCD camera 1 in a container 3. Samples of the dispersion filmsare applied to a transparent substrate such as a PET film or to a blacksubstrate. When a transparent substrate is used, a black background ispreferably chosen. In total, from 2-1 000 samples may be evaluatedsimultaneously. After the samples have been evenly flooded with water 4,the computer-controlled CCD camera 1 begins to take images at regulartime intervals. Even lighting is preferably provided by two diffuselight sources on opposite sides. Increasing blushing results in strongerscattering of light incident on the samples and therefore an increase inbrightness at the sample location. Digital imaging electronics, forexample an image transformer card or a frame grabber interface, are usedto store pictures in digital format in the memory of a control computer9, for example a PC. A self-developed program continuously determines atvariable intervals the average brightness of each of the evaluatedsamples over graphically variable areas of interest which are shown inFIG. 2 by dotted lines. Normalization to a black reference area 5 and awhite reference area 8 allow normalization and the following of thevariation of blushing with time, even at a varying illuminationintensity. The kinetics of blushing of the individual samples may becontinuously updated on the screen, for example by showing appropriatebrightness/time curves. Examples of such curves are shown in FIG. 3. Therepeat frequency and overall duration of the measurement are variable.The inverse process, ie the reduction of blushing after removal ofwater, may also be followed. The brightness/time variations may bedescribed by simple exponential curves. This facilitates thecomparability of different samples with the use of rate constants,characteristic times and/or maximum achievable brightnesses.

1. A process for parallel and automated evaluation of the blushing of nsamples of dispersion films, which comprises: (a) providing anilluminated array of n spatially separated samples of the dispersionfilms, (b) imaging the array of n samples by means of a CCD camera atm+1 different times T₀ to t_(m) and digitizing the images, (c)automatically determining a plurality of brightnesses B(k, t_(i)) foreach sample at different times T_(i) selected from t₀ to t_(m) from them+1 different digitized images for at least a portion of the n samples,where B(k, t_(i)) is the brightness of the kth of n samples at therespective time t_(i) and is a measure for the blushing of this sampleat this time, (d) determining a value P(k) of a parameter which ischaracteristic of the kinetics of the blushing from the differentbrightnesses B(k, t_(i)) for each sample for at least a portion of the nsamples, where P(k) is the value of the characteristic parameter for thekth of the n samples, (e) comparing the value P(k) for individualsamples.
 2. A process as claimed in claim 1, wherein the brightnessesare normalized to a black surface as a zero value of blushing and to awhite surface as a maximum value of blushing.
 3. A process as claimed inclaim 1, wherein the brightnesses of a sample are determined from adigitized image of a sample array by averaging the brightnesses of allpixels which are assigned to a certain area of interest within thissample.
 4. A process as claimed in claim 1, wherein the number n of thesamples is from 2 to 1 000.